In recent years, the reduction of CO2 emissions has been sincerely desired in order to address atmospheric pollution and global warming. The automotive industry has a growing expectation on the introduction of electric vehicles and hybrid electric vehicles for the reduction of CO2 emissions. Under these circumstances, the development of high-performance secondary batteries as motor driving power sources of these vehicles has become an urgent necessity.
As the motor driving secondary batteries where high capacity and good cycle characteristics are required, attentions are being given to lithium ion secondary batteries having high theoretical energy.
In general, the lithium ion secondary battery includes a positive electrode and a negative electrode. The positive electrode has a positive electrode collector and a positive electrode active material and the like applied to both sides of the positive electrode collector, whereas the negative electrode has a negative electrode collector and a negative electrode active material and the like applied to both sides of the negative electrode collector. These positive and negative electrodes are connected to each other via an electrolyte layer and accommodated in a battery case.
It is considered that the selection of the positive and negative electrode active materials of the positive and negative electrodes is of extreme importance to improve the performance characteristics such as capacity characteristics and output characteristics of the lithium ion secondary battery.
There has been proposed a lithium ion secondary battery using a composite oxide represented by: xLi[Mn1/2Ni1/2]O2.yLiCoO2.zLi[Li1/3Mn2/3]O2 (where x+y+z=1; 0<x<1; 0≦y<0.5; and 0<z<1) as a positive electrode active material and a carbon material as a negative electrode active material (see Patent Document 1).
One example of composite oxide usable as the positive electrode active material is of the general formula: aLi[Li1/3M12/3]O2.(1−a)LiM2O2. This composite oxide shows a high discharge capacity of 200 mAh/g and good cycle characteristics and thermal stability and is thus expected to provide good performance as the positive electrode active material.
In order for the battery as a whole to attain high capacity characteristics, it is preferable that not only the positive electrode active material but also the negative electrode active material shows a high capacity. Silicon (Si)-containing negative electrode active materials, which are much higher in capacity than carbon materials, become a focus of attention as such a high-capacity negative electrode active material.
In the case of the battery where the above composite oxide is used as the positive electrode active material in combination with the silicon-containing negative electrode active material, however, there is a problem that the kind of electrolytic solution applicable to the battery is limited due increase in discharge capacity. This is because a lithium salt contained in the electrolytic solution is decomposed by oxidation on the positive electrode side. In the case where the electrolytic solution contains lithium hexafluorophosphate (LiPF6), which is most common as a support electrolyte, hydrogen fluoride (HF) is generated by hydrolysis of LiPF6 and reacts with silicon of the negative electrode so that the negative electrode may deteriorate in performance.